1. Field of the Invention
This invention relates in general to a fritted sparger and more specifically to a fritted sparge device capable of utilizing common test tubes. A coating of anti-foam solution on the inside of the test tubes prevents foaming.
2. Background of the Invention
It is often desirable to determine the presence and nature of volatile organic compounds in water, soil, and sludge. Such a determination is a significant part in the Environmental Protection Agency's monitoring program. Various publications, such as "Test Methods for Evaluating Solid Waste, SW-846", describe methods for making the determination. In general, the method requires placing the samples (aqueous samples or soil/water dispersions) in sparging vessels and passing high purity gas bubbles through the samples. The high purity gas bubbles collect the organic vapor and carry it to an absorption tube that concentrates the vapor for subsequent thermal desorption and analysis by gas chromatography. The Environmental Protection Agency requires that these sparging vessels meet requirements for bubble size, location of bubble origination, and the ability to be heated.
Commercial purge and trap analytical instruments use several types of vessels for holding the sample during sparging. The most common vessel is a specially blown glass tube containing a frit fused into the bottom (hereinafter a "bottom fritted sparger"). Purge gas is introduced into the bottom of the vessel through the frit. Another method of sparging uses a glass cylinder vessel, such as a common test tube, for retaining the sample. A narrow needle inserted into the test tube and terminating near the bottom is used to induce purge gas into the sample. This is called a needle sparger.
During sparging, the gas passes through the frit as a fine bubble froth or through the needle as a line of bubbles. Where practical (see below) fritted sparging produces the most favorable action and results.
A common (and EPA required) method of inducing samples to the vessel is by use of an induction needle, which, in a needle sparger, may be the same needle with appropriate valving. With appropriate valving, this same induction needle may be used to drain the sample from the vessel.
At their interface with the sparging vessel, conventional purge and trap analysis devices provide a source of purge gas and means for introducing it to the sample (i.e. the needle sparger or glass frit), include a tube fitting means for enclosing the top of the vessel including means for collecting the gas emitted from the vessel, and include an induction needle usually passing through the top enclosure.
Bottom fritted spargers are expensive ($50-$90 at present) and are easily broken. Keeping a supply on hand to allow for breakage ties up many hundreds of dollars. Exacerbating this money problem, is the fact that different vessels are required by various analyzer manufactures. Thus, several inventories may be required.
Bottom fritted spargers must be cleaned between samples that have an oily, soapy, or silty character. The cleaning requires solvents, high purity water, and oven baking. Additionally, cleaning is quite time consuming and increases the likelihood of breakage.
Use of a multiple array autosample analyzer dictates that some samples sit in the vessel for a long period of time. The long contact time causes settling and the sample may clog the frit which may prevent successful sparging. This often occurs with common soil/water samples.
Needle spargers allow use of common disposable test tubes. Needle spargers are not as subject to clogging as are bottom fritted spargers. However, even needle spargers are subject to clogging when samples, such as soil samples, sit for more than an hour, such as in a multiple array autosampler. Also, when the test tube containing the sample is inserted into the tube fitting, the needle often cores the soil sample and clogs as result.
Frequent switching between bottom fritted spargers and needle spargers is common but time consuming. In addition to the obvious hardware manipulation, it requires increased recalibration steps.
Another method uses a fritted glass dip tube in place of a needle sparger. Common disposable test tubes can be used with this method. The large diameter required of the fritted glass dip tube precludes the use of the smaller (and cheaper) test tubes because the dip tube takes up a great portion of the volume. The large bore of the glass dip tube, its hyrophobic nature, and its inherently large porosity combine to lead to backfilling of the tube upon standing. When very small particles of soil are present, the tube tends to clog and the resulting backpressure prevents successful sparging. Backfilling may lead to cross contamination. Conventionally, the fritted dip tube replaces the induction needle. Hence, one must fill the test tube with a liquid sample before attaching the test tube to the analyzer. This promotes ambient contamination, loss of very volatile constituents, and violates EPA protocol. Special fittings are required to switch between use of the fritted glass dip tube and normal bottom fritted spargers.
Laboratory throughput is greatly increased by automatic sparger filling from a multiple vial sampler. This technique requires an automatic sparger drain feature which is not usable with either needle spargers or a fritted drip tube.
Therefore, it is desirable to have an improved sparging apparatus for purging and trapping which overcomes the shortcomings of the conventional art.
It is particularly preferable that the sparging assembly utilize common disposable test tubes.